Resonant micromachined biochemical sensor

Abstract

A sensor system is formed from a micro machined resonant structure with multiple resonant elements, a tracking resonator control electronics, and signal processing algorithms. The moving elements of the resonator are coated with chemically active materials that change mass when exposed to the target chemical resulting in a change in frequency or period of oscillation. The changes in frequency or period are processed by multi-sensor chemical detection algorithms to identify chemical types and concentrations. In essence, the resonator and drive electronics form a closed loop oscillator operating at the resonator's natural frequency. The resonators are formed from silicon using photolithographic processes. The resonator design includes in-plane resonant motion combined with dynamic balance to operate with a high Q even in the presence of atmospheric pressure.

What is claimed is: 
     
       1. A system for detecting one or more target chemical compounds, comprising:
 an array of one or more resonant sensors each comprising:
 a mechanical spring-mass system employing:
 a thin planar structure lying in a plane and having a damped natural frequency and comprising:
 two motional masses spaced apart along an X-axis in the plane and dynamically linearly balanced along the X-axis and suspended by two or more spring suspension elements extending in a Y-axis in the plane from the masses to a surrounding frame, with the spring suspension elements having a cross-sectional geometry that results in substantially higher stiffness to out of plane motion than X-axis motion to constrain the masses to substantially linear resonant motion in the plane of the thin planar structure while minimizing motion out of the plane, the masses further being connected by a linkage disposed therebetween constraining them to move linearly 180 degrees out of phase with each other along the X-axis and substantially constraining both masses from moving together side-to-side along the X-axis, wherein the linkage comprises flexible beams that connect both to the frame and to each motional mass, and wherein the flexible beams define an elongated diamond shape having a first pair of opposite apexes connected to the motional masses along the X-axis and a second pair of opposite apexes connected to the frame and spaced apart farther than the first pair; and 
 an active coating on or in the motional mass with an affinity to capturing or reacting with the target chemical compound in order to change its own mass in response to the presence of the target chemical compound and affect a change in the damped natural frequency of the thin planar structure; 
 
 
 drive electronics to control the amplitude of a driven resonant mode of each sensor in the array at its damped natural frequency; 
 a signal processing system to detect a change in a damped natural frequencies or periods of the one or more sensors; and 
 a processor including a memory possessing a set of algorithms to convert the detected change to an indication of target chemical detection. 
 
 
     
     
       2. The sensor system of  claim 1 , wherein each spring suspension element comprises multiple thin flat beams having a length in the Y-direction a height in the Z-direction substantially greater than a thickness on the X-direction. 
     
     
       3. The sensor system of  claim 1 , wherein the signal processing system receives input from a sensing surface area in which the sensing surface area-to-motional mass ratio is improved by forming holes comprising:
 a plurality of hexagonally shaped holes through the thin planar structure in a normal direction to the motion of the sensors. 
 
     
     
       4. The sensor system of  claim 1 , wherein the signal processing system receives input from a sensing surface area in which the sensing surface area-to-motional mass ratio is improved by using an open porous structure in the sensing area. 
     
     
       5. The sensor system of  claim 1 , wherein the array has multiple sensors with one or more having different active coating types, and further including:
 electronics to provide chemical detection and compound identification based upon a signature of frequency changes for the different active coating types. 
 
     
     
       6. The sensor system of  claim 5 , wherein:
 the multiple sensors are slightly offset in frequency to allow simultaneous operation with reduced potential for cross coupling. 
 
     
     
       7. The sensor system of  claim 5 , wherein:
 one or more substantially identical sensors have a coating with a substantially different mass capture response to serve as a reference resonator wherein a chemical detection signal is primarily the difference between an active resonator frequency or period shift and a reference resonator frequency or period shift. 
 
     
     
       8. The sensor system of  claim 5 , utilizing heating of the motional masses to slowly raise the temperature of a chemical sensing element containing a concentrated sample above the dew point while resonating at its resonant frequency to detect evaporating chemical mass based upon its vapor pressure as a function of temperature. 
     
     
       9. The sensor system of  claim 1 , wherein the motional mass comprises a large portion of porous silicon to achieve high surface area to mass ratio. 
     
     
       10. The sensing system of  claim 1 , further including a magnetic field substantially orthogonal to the one or more conductive traces and a vector of resonant motion to provide drive force to sustain resonant amplitude by running current through each conductive trace, in which the magnetic field comprises a permanent magnet magnetized through its thickness with one polarity backed by a highly permeable flux material which enhances the flux density at the sensor, both of which are in close proximity to the sensor with the magnet facing the sensor on one side with a small air gap and having a first dimension essentially parallel to the plane of the sensor and a second dimension extending sufficiently far to expose the conductive traces to the magnetic field such that the flux return path after the flux passes through the sensor does not require another magnet. 
     
     
       11. A system for sensing one or more target chemical compounds, comprising:
 an array of one or more resonant sensors each comprising:
 a mechanical spring-mass system employing:
 a thin planar structure lying in a plane and having a damped natural frequency and comprising:
 a motional mass comprising two motional masses spaced apart and dynamically linearly balanced along an X-axis in the plane, wherein two or more spring suspension elements extend in a Y-axis in the plane from the masses to a surrounding frame, the spring suspension elements having a cross-sectional geometry that results in substantially higher stiffness to out of plane motion than X-axis motion to constrain the masses to substantially linear resonant motion in the plane of the thin planar structure while minimizing motion out of the plane; and 
 an active coating on or in the motional mass with an affinity to capturing or reacting with the target chemical compound in order to change its own mass in response to the presence of the target chemical compound and affect a change in the damped natural frequency of the thin planar structure; 
 
 a linkage constraining the two dynamically-balanced motional masses to move 180 degrees out of phase with each other along the X-axis and substantially constraining both masses from moving side-to-side together, wherein the linkage comprises flexible beams that connect both to the frame and to each motional mass, and wherein the flexible beams define an elongated diamond shape having a first pair of opposite apexes connected to the motional masses along the X-axis and a second pair of opposite apexes connected to the frame and spaced apart farther than the first pair; 
 
 
 drive electronics to control the amplitude of the driven resonant mode of each sensor in the array at its damped natural frequency; 
 an electromagnetic drive and velocity sensor comprising one or more conductive traces across the motional mass oriented substantially orthogonal to a driven resonant motion; 
 a magnetic field substantially orthogonal to the one or more conductive traces and a vector of resonant motion to provide drive force to sustain resonant amplitude by running current through each conductive trace, and 
 one or more separate conductive traces on the motional mass that are substantially orthogonal to the vector of resonant motion to measure back EMF signals for velocity detection. 
 
     
     
       12. The sensing system of  claim 11 , in which sensor motion is controlled closed-loop by providing drive signals that are phase and frequency coherent to synchronously drive signals that control a desired amplitude of motion at resonance, and wherein a means of driving the signals closed loop are by waveform width modulation. 
     
     
       13. The sensing system of  claim 11 , in which sensor motion is controlled closed loop by providing drive signals that are phase and frequency coherent to synchronously drive signals that control a desired amplitude of motion at resonance, and wherein a means of driving the signals closed loop are by pulse width modulation. 
     
     
       14. The sensing system of  claim 11 , in which a sensor drive is in closed-loop-control, a waveform width control off command leaves a sensor drive loop electrically open so as to not produce current losses from back EMF related loss paths. 
     
     
       15. The sensing system of  claim 11 , in which a peak back EMF which is compensated for sensor frequency and temperature effects is compared with a desired peak back EMF and the difference is used to form an error signal for control in a processing element, and the error signal is processed by a controller to form a correction signal, and an error signal command is adjusted to maintain a desired sensor displacement. 
     
     
       16. The sensing system of  claim 11 , in which a sensor initial motion is excited by a sweep frequency forcing function;
 the system further including a sensing subsystem by which a synchronous motion is detected including a combination of sensor back EMF for velocity measurement and period counting for discrimination that a resonance period has been detected. 
 
     
     
       17. The sensing system of  claim 11 , wherein the resonant amplitude has been established and an amplitude sensing is measured during cycles wherein a drive current is disabled. 
     
     
       18. The sensing system of  claim 11 , wherein the drive current is disabled in the regions of the cycle near zero amplitude to reduce the noise in the velocity signal. 
     
     
       19. The sensing system of  claim 11 , in which sensor frequency is measured by electronics which amplify the essentially sinusoidal back EMF velocity signal and utilizes a comparator to convert to a square waveform for frequency or period measurement using the number of counts of a high frequency reference clock. 
     
     
       20. The sensing system of  claim 11 , in which an analog-to-digital convertor is used to sample back EMFs of multiple sensors in an array and provide a sample to a processing element, the processing element utilizing a digital phase-locked-loop establishes the critical timing information from comparator inputs to determine when in the sample the peak value is to occur. 
     
     
       21. The sensing system of  claim 11 , in which sensor frequency is measured by electronics which amplifies an essentially sinusoidal back EMF velocity signal and rectifies the signal to measure velocity. 
     
     
       22. The sensing system of  claim 11 , in which sensor frequency is measured by electronics which amplifies an essentially sinusoidal back EMF velocity signal and detects the peak velocity with a peak detector. 
     
     
       23. The sensing system of  claim 11 , in which the magnetic field comprises a permanent magnet magnetized through its thickness with one polarity backed by a highly permeable flux material which enhances the flux density at the sensor, both of which are in close proximity to the sensor with the magnet facing the sensor on one side with a small air gap and having a first dimension essentially parallel to the plane of the sensor and a second dimension extending sufficiently far to expose the conductive traces to the magnetic field such that the flux return path after the flux passes through the sensor does not require another magnet.